Delivery
Disruptions

For
gene therapy to be successful, literally billions of replacement
genes need to be delivered, and they need to be delivered to just
the right place. For example, if you're trying to cure a disease
in the liver, you don't want genes winding up in the bloodstream,
you've missed the target. Scientists are still working on a way
to guarantee the genes get exactly where they need to go.

Another obstacle
is controlling how the gene functions. Even if the genes get to
their target,
"different genes function at different intensities, much like
a light on a dimmer switch. Scientists have yet to figure out how
much to turn a gene off or on. Too little and there's no
benefit to the patient. Too much and there can be side effects."*

The final obstacle
is rejection. Just like a body can reject an organ transplant, a
patient's immune system may attack the new genes, "rejecting"
them. If the genes are rejected, they have no opportunity to have
a lasting effect.